In line with an increase in technology development of and demand for mobile devices, demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries which have high energy density and discharge voltage have been extensively studied and are commercially available and widely used.
Generally, a lithium secondary battery has a structure in which an electrode assembly consisting of a positive electrode, a negative electrode, and a separator is impregnated with a lithium electrolyte, and the positive or negative electrode is manufactured by coating an electrode current collector with a positive or negative electrode slurry.
The positive electrode slurry and the negative electrode slurry include electrode mixtures including: a lithium transition metal oxide and a carbon-based active material, respectively, as an electrode active material for storing energy; a conductive material to impart electrical conductivity; and a binder that adheres the positive or negative electrode slurry to a current collector to impart adhesion therebetween, N-methylpyrrolidone (NMP), and the like. In this case, copper (Cu) foil, aluminum (Al) foil, or the like is generally used as the electrode current collector.
However, in the manufacture of such electrode, adhesion between the electrode mixture and the current collector is reduced in a pressing process or the subsequent manufacturing process and, as a result, dust particles or the like may be generated. In addition, during operation of a battery, problems such as peeling of the electrode active material attached to a surface of the current collector due to interfacial resistance between the current collector and the electrode slurry may occur. Such reduction in adhesion and peeling of an active material due to this result in an increase in internal resistance of a battery, thus causing significant deterioration of battery performance such as deterioration of output characteristics, a decrease in battery capacity, and the like.
To address these problems, a method of increasing adhesion between a current collector and an electrode active material by forming fine irregularities at a surface of the current collector via etching has been proposed. However, this method is advantageous in that a current collector having a high specific surface area can be obtained by a simple process, while having a problem such as a reduction in the lifespan of the current collector due to the etching treatment.
As another alternative, a method of increasing adhesion between a positive or negative electrode current collector and an electrode active material by coating a surface of the positive or negative electrode current collector with a silane-based coupling agent, or forming an anchor film on a positive or negative electrode current collector using a coating solution including a conductive material, an adhesive resin, and an alcohol has been proposed. However, these methods are advantageous in that high adhesion is obtained between the current collector and the active material, while having a problem such as deterioration of battery performance due to high internal resistance.
Therefore, there is an urgent need to develop a method of increasing adhesion between a current collector and an electrode mixture and also improving output characteristics by reducing internal resistance of a battery.